The present invention relates to an insulating material for coaxial cables, a coaxial cable having a foam insulating layer obtained from said material, and a method for preparing said coaxial cable. More particularly, the present invention relates to an insulating material preferably used for forming a foam insulating layer of coaxial cables, a coaxial cable having a foam insulating layer prepared from said material, and a method for producing said cable.
Traditionally, insulating wires having a foam insulating layer on conductors, which is formed by foam extrusion of a polymer composition, have been widely used.
The polymer composition used for such foam extrusion comprises, for example, a polyolefin resin such as polyethylene, a thermally decomposable compound such as 4,4xe2x80x2-oxybisbenzenesulfonylhydrazide (OBSH) and azodicarbonamide (ADCA) as a nucleator, and a foaming agent such as chlorofluorocarbon.
When a composition containing a polyolefin resin and OBSH as a nucleator is used, however, water is produced during thermal decomposition of OBSH and remains in foamed articles, which in turn adversely affects transmission performance and the like. To avoid this, the obtained foam insulating layers need to be dried by, for example, vacuum drying, which is problematic from industrial aspects.
When a composition containing ADCA as a nucleator is used, moreover, a residue from thermal decomposition of ADCA, such as biurea, cyanuric acid and urazol, is produced to cause dielectric loss in high frequency regions due to the polarization thereof, which noticeably increases the dissipation factor (hereinafter to be referred to as tan xcex4 of the obtained foam insulating layer, thereby degrading the electric property of the insulating layer. The decomposition residue also causes high water absorption of the foam insulating layer, and the foamed article therefore requires storage in a low humidity atmosphere.
Japanese Patent Unexamined Publication No. 348141/1992 discloses a composition for foam extrusion which comprises a base resin containing a polyolefin resin capable of heat-melt extrusion, and a low polarity boron compound powder non-decomposable at a foaming temperature as a nucleator.
Japanese Patent Application No. 12983/1995 discloses a composition for foam extrusion which comprises a base resin containing a polyolefin resin capable of heat-melt extrusion and a specific boron nitride as a nucleator.
The composition containing boron nitride provides a foamed article far superior to foamed articles obtained by conventional foamable compositions containing a known nucleator such as ADCA, in various properties, specifically electric property (e.g., tan xcex4). Yet, such foamed article also has defects in that it has larger cell diameter than in the foamed articles obtained from conventional materials to disqualify itself for such use as requires mechanical strength or hardness, and that it is costly.
Japanese Patent Unexamined Publication Nos. 279739/1990 and 64335/1991 teach a method for producing a foamed article which comprises heating a mixture of a thermoplastic resin and a fluorocarbon powder or a nucleator for plasticization, and adding a foaming agent.
The above-mentioned two gazettes relate to a foamed article for thermal insulation and do not suggest a foamable composition (insulating material) suitable for forming a foam insulating layer of coaxial cables, which is capable of producing a foamed article particularly superior in electric properties, and a coaxial cable having an insulating layer obtained from said composition.
It is therefore an object of the present invention to provide an insulating material for economical coaxial cables, which is capable of producing foamed articles having fine and uniform cells and a high expansion ratio, and which obviates drying steps of foamed articles.
Another object of the present invention is to provide a coaxial cable having an insulating layer superior in electric properties, which layer being obtained from said insulating material.
A still another object of the present invention is to provide a method for producing the above-mentioned coaxial cable.
According to the present invention, there have now been provided the following insulating materials (1)-(6) for coaxial cables, coaxial cables (7)-(12), and a method (13) for producing coaxial cables.
(1) An insulating material for coaxial cables, comprising a polyolefin resin capable of heat-melt extrusion, and a fluororesin powder as a nucleator, which material being capable of foam extrusion in the presence of a foaming agent.
(2) The insulating material of the above (1), further comprising a foaming agent.
(3) The insulating material of the above (1) or (2), wherein the polyolefin resin is a high density polyethylene, a low density polyethylene or a mixture of a high density polyethylene and a low density polyethylene.
(4) The insulating material of any one of the above (1) to (3), wherein the fluororesin powder is a member selected from the group consisting of polytetrafluoroethylene powder, tetrafluoroethylene-perfluoroalkylvinylether copolymer powder and ethylene-tetrafluoroethylene copolymer powder.
(5) The insulating material of any one of the above (1) to (4), wherein the fluororesin powder is added in a proportion of 0.01-10 parts by weight relative to 100 parts by weight of the polyolefin resin.
(6) The insulating material of any one of the above (1) to (5), wherein an average particle size of the fluororesin powder is 0.1-100 xcexcm.
(7) A coaxial cable comprising a foam insulating layer prepared from an insulating material for coaxial cables, which material comprising a polyolefin resin capable of heat-melt extrusion and a fluororesin powder as a nucleator, and being capable of foam extrusion in the presence of a foaming agent.
(8) The coaxial cable of the above (7), wherein the insulating material for coaxial cable contains a foaming agent.
(9) The coaxial cable of the above (7) or (8), wherein the polyolefin resin is a high density polyethylene, a low density polyethylene or a mixture of a high density polyethylene and a low density polyethylene.
(10) The coaxial cable of any one of the above (7) to (9), wherein the fluororesin powder is a member selected from the group consisting of polytetrafluoroethylene powder, tetrafluoroethylene-perfluoroalkylvinyl-ether copolymer powder and ethylene-tetrafluoroethylene copolymer powder.
(11) The coaxial cable of any one of the above (7) to (10), wherein the fluororesin powder is added in a proportion of 0.01-10 parts by weight relative to 100 parts by weight of the polyolefin resin.
(12) The coaxial cable of any one of the above (7) to (11), wherein an average particle size of the fluororesin powder is 0.1-100 xcexcm.
(13) A method for producing a coaxial cable, comprising extruding, on a conductor, a composition comprising a polyolefin resin capable of heat-melt extrusion, and a fluororesin powder as a nucleator, in the presence of a foaming agent to form a foam insulating layer.